This invention relates, in general, to completion, production or injection trees for producing oil and gas from a subsea well and is specifically directed to a completion tree having a new and improved manifold control system.
Although a typical prior art completion tree will be described in more detail in the Detailed Description hereinafter, for the purposes of understanding the background to the invention, suffice to say here that, typically a completion tree is equipped to be locked onto the wellhead of a subsea well and comprises essentially a series of valves, sometimes with valve operators, for connecting production and other tubing within the well to subsea flowlines which bring the well production to the production platform or to shore. A tree manifold at the top of the tree provides the junction point for all hydraulic control functions and interfaces with a tree cap during production and with a running tool during installation of the tree and re-entry to the well for workover. Both the tree cap and the running tool are equipped with stingers or stabs which enter pockets in the tree manifold and, through the stinger, direct the flow of hydraulic fluid (control signals) to various valve operators, the flowline connector, the wellhead connector, etc. The number of valve operators in the tree depends on; the tubing program, the number of stingers, whether or not the tree is to be a through flowline (TFL) type, whether or not access to the annulus is to be provided, and whether or not special features are required, such as chemical injection, etc.
Usually the completion tree is assembled at the surface and landed at the wellhead with the running tool being connected at the tree manifold. The running tool provides surface operated control lines with access to the wellhead connector for hydraulically actuating and connecting the tree onto the wellhead and also allows other functions to be accomplished, including access to the production tubing, the annulus tubing through control of the tree valves and control of subsea safety valve(s) conventionally located 50-200 feet below the tubing hanger.
After the completion tree is in place and connected to the wellhead, the running tool is disconnected, brought to the surface and the tree cap is connected thereto. Thereafter the running tool and tree cap are lowered so that the tree cap is then connected to the tree manifold. It is the tree cap that directs the hydraulic control signals from a bundle of control lines (umbilical) connected to the tree cap to the various valve operators. This function of the tree cap is sometimes referred to as a U-function since the signals from one port in the tree manifold are directed through the tree cap to another port in the manifold to the valve operators.
During production, the tree cap and normally a tree cap protector are in place on the manifold and production is out the side of the tree to the subsea flowlines.
If re-entry to the well for workover is desired, the running tool is lowered to remove the tree cap protector and tree cap and after bringing them to the surface is again run and stabbed on to the tree manifold to control the tree valve operators from the surface to perform whatever work is necessary to the well.
A major disadvantage with the present system as above described is that when the tree cap is removed from the tree manifold and before the running tool can be landed, seawater intrusion and contamination of the control circuits can result. Also, during the periods when neither the tree cap nor the running tool are connected to the tree manifold, the completion tree is completely divorced from the surface and thus out of control. Further, the number of running operations involving costly rig time required for closure of the production tree with a tree cap providing the U-function and subsequent retrieval for access is higher than required with this invention.